Hydrostatic bearing for linear motion guidance

ABSTRACT

A self-compensating hydrostatic (pressurized fluid film) linear bearing that maintains a fluid gap between a carriage and a rail when relative forces are applied. The geometric shape of the rail and mating carriage enable the bearing to have very high stiffness and load capacity without exessive detrimental carriage deformation. The carriages contain bearing grooves and lands which control and use fluid pressure to provide a very high degree of restoring force in response to changes in the fluid gap. The fluid emanating from the bearing gap is prevented from immediately leaking from the bearing carriage, and is instead routed back to the source from which it is pumped, thereby sealing the bearing carriage and simplifying the handling of the lubricating fluid. The hydrostatic bearing is particularly designed to be compact and to be bolt-for-bolt compatible with conventional linear bearings.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/617,390, filed Jul. 11, 2003, and claims priority from U.S.Provisional Patent Application No. 60/406,933, filed on Aug. 30, 2002,the entire contents of both of which are hereby incorporated into thisapplication by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to mechanical bearings and, more particularly, tohydrostatic bearings for linear motion guidance.

2. Description of Related Art

A linear bearing typically includes a carriage and a rail slideablymounted on the carriage. A component, such as a moveable portion of amachine tool, is typically removably mounted on the carriage for slidingmovement along the rail with the carriage. A conventional linear bearinguses rolling elements or polymer linings to reduce friction between thecarriage and rail.

In a hydrostatic linear bearing, lubricating fluid is pumped into thecarriage and rail at high pressures so that a thin film of lubricant ismaintained between the carriage and rail as the carriage slides alongthe rail, even when large loads are applied to the carriage and rail.The lubricating fluid flows into shallow cavities and channels providedin the carriage and rail. These cavities in the carriage and rail aresometimes referred to as bearing pockets.

In order to maintain the thin fluid film between the carriage and therail, some fluid flow resistance or compensation must be provided in thebearing. Typically, capillary tubes, orifices, and control valves areused to provide the required resistance or compensation. A hydrostaticbearing may also be of the self-compensating type, in which resistivelands in the bearing pockets (i.e., planar areas over which fluid flowis restricted), or other bearing pocket features, are used to providethe required flow resistance or compensation.

Hydrostatic bearings a very desirable in a number of applicationsbecause they generally have very high stiffness, high load capacity, lowfriction, no wear, high damping, and resistance to contamination. All ofthese advantages make hydrostatic bearings particularly desirable inmachine tool applications, where linear bearings with high rigidity anddamping capabilities are needed to enable very precise motion that isfree of excessive vibration.

Despite their advantages, hydrostatic bearings have not been widely usedin the machine tool industry due to a number of practical problems withtheir installation and use. For example, the typical compensatingdevices, orifices, and control valves are often too difficult to installproperly in machine tools, and may also be delicate, expensive, or tooprone to contamination to provide a reasonable useable lifetime.Additionally, the fluid used for lubrication is easily contaminated bychips and coolant used in the machining process. For these reasons,linear bearings based on rolling elements have been used predominantlyin the machine tool industry.

SUMMARY OF THE INVENTION

One aspect of the invention relates to a self-compensating hydrostaticbearing. The self-compensating hydrostatic bearing includes a bearingrail and a bearing carriage constructed and arranged to be mounted forhydrostatically supported movement on the bearing rail. The bearingcarriage includes a plurality of self-compensating bearing pads providedon surfaces that oppose the bearing rail. The bearing pads areconstructed and arranged to be in fluid communication with one anotherand with a pressurized fluid source.

End sealing structures are provided on end portions of the bearingcarriage. At least one edge of the end sealing structures engages thebearing rail to prevent hydrostatic fluid from leaking from between thebearing carriage and the bearing rail. Side sealing structures areprovided on side portions of the bearing carriage and extend at least aportion of the length of the bearing carriage. At least one edge of theside sealing structure engages the bearing rail to prevent hydrostaticfluid from leaking from between the bearing carriage and the bearingrail.

The bearing also includes a fluid return system provided within portionsof the bearing carriage that are sealed by the end and side sealingstructures. The fluid return system is constructed and arranged to routefluid towards the pressurized fluid source.

Another aspect of the invention relates to a self-compensatinghydrostatic bearing. The bearing includes a bearing rail having at leastone substantially contiguous support surface constructed and arranged tosupport the hydrostatic bearing and a bearing carriage constructed andarranged to be mounted for hydrostatically supported movement on thebearing rail.

The bearing carriage includes a plurality of self-compensating bearingpads provided on surfaces that oppose the bearing rail. The bearing padsare constructed and arranged to be in fluid communication with oneanother and with a pressurized fluid source. Sealing structure isprovided on portions of the bearing carriage. At least one edge of thesealing structure engages the bearing rail to prevent hydrostatic fluidfrom leaking from between the bearing carriage and the bearing rail. Thebearing carriage also includes a fluid return system provided withinportions of the bearing carriage that are sealed by the sealingstructure. The fluid return system is constructed and arranged to routefluid towards the pressurized fluid source.

A further aspect of the invention relates to a bearing carriage thatcomprises one or more bearing pads and a fluid recovery system. Thebearing pads are constructed and arranged to receive fluid from apressurized fluid source and to cause that fluid to flow selectivelyover a collection of bearing grooves and resistive lands so as to createa supporting fluid layer between the bearing carriage and a structure onwhich the bearing carriage is mounted for movement.

The fluid recovery system is constructed and arranged to prevent fluidfrom flowing out of the space between the bearing carriage and thestructure on which the bearing carriage is mounted for movement and toroute the fluid back towards the pressurized fluid source. The fluidrecovery system includes sealing structure having contiguous end andside portions. The end portions are constructed and arranged to sealends of the bearing carriage and the side portions are constructed andarranged to extend along at least a portion of sides of the bearingcarriage to seal the sides. The end portions include a double-lippedseal. A first lip of the double-lipped seal engages the structure onwhich the bearing carriage is mounted for movement and the second lip ofthe double-lipped seal prevents debris from entering the bearingcarriage. The fluid recovery system also includes reservoir structuredefined by portions of the bearing carriage and sealed by the sealingstructure and drain grooves constructed and adapted to conductpressurized fluid from the bearing pads to the reservoir structures.

Further aspects of the invention relate to machine tools or portionsthereof mounted on hydrostatic bearings.

Yet another aspect of the invention relates to a bearing carriage. Thebearing carriage comprises one or more bearing pads constructed andarranged to receive fluid from a pressurized fluid source and to causethat fluid to flow selectively over a collection of bearing grooves andresistive lands so as to create a supporting fluid layer between thebearing carriage and a structure on which the carriage is mounted formovement.

The bearing carriage also includes a fluid recovery system constructedand arranged to prevent fluid from flowing out of the space between thebearing and the structure on which the bearing carriage is mounted formovement and to route the fluid back towards the pressurized fluidsource. The fluid recovery system includes a sealing structure havingcontiguous end and side portions. The end portions are constructed andarranged to seal ends of the bearing carriage. The side portions areconstructed and arranged to extend along at least a portion of the sidesof the bearing carriage to seal the sides. The end portions include adouble-lipped seal. A first lip of the double lipped seal engages thestructure on which the bearing carriage for movement, and a second lipof the double-lipped seal prevents debris from entering the bearingcarriage.

The bearing carriage also includes reservoir structures defined byportions of the bearing carriage and sealed by the sealing structure anddrain grooves constructed and arranged to conduct pressurized fluid fromthe bearing pads to the reservoir structures.

Another further aspect of the invention relates to a hydrostaticbearing. The hydrostatic bearing comprises a bearing rail and a bearingcarriage constructed and arranged to be mounted for hydrostaticallysupported movement on the bearing rail. The bearing carriage includesone or more bearing pads provided on surfaces that oppose the bearingrail. The bearing pads are constructed and arranged to be in fluidcommunication with a pressurized fluid source.

The bearing carriage also includes seal receiving grooves and a sealingstructure having contiguous end and side portions. At least a portion ofthe sealing structure is adapted to be received in the seal receivinggrooves. End portions of the sealing structure include double-lippedseals.

A fluid return system is also included in the bearing carriage. Thefluid return system includes a plurality of drain grooves in fluidcommunication with the bearing pads. At least some of the plurality ofdrain grooves are positioned between the bearing pads and the sideportions of the sealing structure.

Yet another further aspect of the invention relates to a method ofsealing a hydrostatic bearing carriage. The method comprises causing orallowing hydrostatic fluid to flow from hydrostatic bearing padsprovided in the bearing into drain grooves provided along the sides ofthe bearing carriage. The method also involves preventing leakage fromthe drain grooves by positioning sealing structures along the sides ofthe bearing carriage so as to capture hydrostatic fluid flowing out fromthe drain grooves, collecting the hydrostatic fluid in a reservoirprovided as a portion of the hydrostatic bearing carriage, preventingthe hydrostatic reservoir from leaving the reservoir except throughdesignated outlets using a first portion of an end sealing structure,and preventing debris from entering the bearing carriage using a secondportion of the end sealing structure.

An additional aspect of the invention relates to a hydrostatic bearingpad. The hydrostatic bearing pad comprises a compensating groove, anadjacent pocket groove enclosing a first planar area therein, and asecond planar area interposed between the compensating groove and theadjacent pocket groove. The first and second planar areas areconstructed and arranged to resist the flow of pressurized fluid whenthe bearing pad is in a load supporting position relative to anothersurface. The bearing pad does not include grooves between thecompensating groove and the pocket groove.

Other additional aspects of the invention relate to self-compensatinghydrostatic bearings having bearing pads as described in the precedingparagraph.

These and other aspects, features and advantages of the invention willbe described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the following drawingfigures, in which like numerals represent like features throughout thefigures, and in which:

FIG. 1 is a perspective view of a hydrostatic bearing in accordance withthe invention without end caps or seals installed;

FIG. 2 is a side elevational view of the carriage of FIG. 1;

FIG. 3 is a schematic diagram of the vertical bearing pads in thecarriage of FIG. 1;

FIG. 4 is a fluid circuit diagram showing the resistances of the bearingpads of FIG. 3;

FIG. 5 is a schematic diagram of the horizontal bearing pads of thecarriage of FIG. 1;

FIG. 6 is a fluid circuit diagram showing the resistances of the bearingpad of FIG. 5;

FIG. 7 is another perspective view of the hydrostatic bearing of FIG. 1,with end caps and seals installed;

FIG. 8 is a sectional view through Line 8-8 of FIG. 7 showing thereservoir end caps and end seals of the hydrostatic bearing;

FIG. 9 is a close-up sectional view of a portion of the structure shownin FIG. 8, showing the end caps and seals in more detail;

FIG. 10 is a sectional elevational view of the carriage of FIG. 1illustrating the side seals;

FIG. 11 is a close-up sectional view of a portion of the structure shownin FIG. 10 in more detail;

FIG. 12 is a side elevational view showing a machine tool tablesupported on several hydrostatic bearings of the type shown in FIG. 1;

FIG. 13 a perspective view showing the underside of the bearing carriageof FIG. 1;

FIGS. 14 and 15 are perspective views of the keeper portions of thebearing carriage of FIG. 1;

FIG. 16 is a perspective view of the side and end seals of the bearingcarriage of FIG. 1 in isolation without the bearing carriage itself;

FIG. 17 is a close-up perspective view of a portion of the side and endseals shown in FIG. 16, illustrating the engagement of the side and endseals; and

FIG. 18 is a schematic perspective view of several hydrostatic bearingsaccording to the invention connected to a hydraulic power unit.

DETAILED DESCRIPTION

FIG. 1 is a perspective view of a hydrostatic linear bearing, generallyindicated at 10, according to the present invention. The bearing 10 iscomprised of a carriage 12 that is mounted for sliding, hydrostaticallysupported movement along a rail 14. The direction of movement is shownby arrow M in FIG. 1.

In the embodiment shown in FIG. 1, the rail 14 has a “T shaped” crosssection. The carriage 12 has a central portion 16 and two keepers, 18A,18B that are clamped or bolted to the central portion 16 of the carriage12. Alternatively, the carriage 12 may be fabricated as a single-piecestructure; however, the use of the two separable keepers 18A, 18B makesthe carriage 12 easier to fabricate, and, in particular, easier tofinish grind. If the carriage 12 is fabricated as a single-piecestructure, special finish grinding equipment may need to be used.

The carriage 12 also includes a number of drain grooves 106, 108A, 108B,110A, 110B, 112A, and 112B extending substantially the entirety of thelength of the carriage. The drain grooves 106, 108A, 108B, 110A, 110B,112A, and 112B will be described in more detail below.

The carriage 12 and rail 14 have rectilinear cross-sections in thisembodiment of the invention. (The term “rectilinear,” as used herein,refers to any shape comprised of line segments without substantialcurvature between adjacent segments.) Although rectilinear crosssectional shapes are generally preferred because they are easier tomachine, the carriage and rail of a hydrostatic bearing according to theinvention may have any desired cross sectional shape. More generally,the carriage 12 may be shaped to engage a rail of substantially anycross-sectional shape.

As shown in FIG. 1, the rail 14 includes drilled and counterbored holes20 that are used to secure it to a machine tool bed or other rigidstructure. The carriage 12 includes drilled and tapped holes 22 suchthat raised surfaces 24A, 24B, 24C may be clamped rigidly to the matingsurface of a machine tool table or other structure that requires linearmotion guidance. (The use of the hydrostatic bearing 10 will bedescribed in more detail below.)

In general, the overall size and shape of the carriage 12 and rail 14,and the locations of the holes 20, 22 in the rail and carriage may beselected so as to be “bolt-for-bolt” compatible with and of the samesize as standard rolling element linear bearings. It is advantageous ifthis type of compatible configuration is used, because a hydrostaticbearing 10 according to the invention may then be directly substitutedfor a rolling element-type linear bearing in an existing machine tool ortool design.

FIG. 2 is a side elevational view of the carriage 12. The carriage 12 ishydrostatically supported by a number of bearing pads provided ininterior surfaces of the carriage 12. The locations of the verticalbearing pads 26A, 26B, 28A, 28B and the horizontal bearing pads 30A, 30Bare also shown in the perspective views of FIGS. 13-15 and will bedescribed in more detail below with respect to those figures. (The terms“vertical” and “horizontal,” as used with respect to the bearing pads,refer to the direction of the applied loads that the respective bearingpads resist.) Fluid pressure exerted through the bearing pads 26A, 26B,28A, 28B maintains the bearing carriage 12 at a small distance from thebearing rail 14. Typically, the clearance between the bearing pads 26A,26B, 28A, 28B, 30A, 30B and the rail 14 would be on the order of about0.001 inches to about 0.005 inches.

In this description, the terms “fluid” and “hydrostatic fluid” are usedinterchangeably to refer to any fluid that may be used in a bearing 10according to the present invention. Many such fluids are known in theart, including hydrocarbon-based oils, silicone-based oils, water,water-based compositions, and air or another suitable gas. In machinetool applications, hydrocarbon-based oils may be preferred for someapplications. These oils tend to reduce or eliminate corrosion problems,and may also have relatively high viscosities, which help to reduce thebearing flow rate and associated pumping power needed to pressurize thebearing 10.

Water-based hydrostatic fluids also have certain advantages and may alsoserve in hydrostatic bearings 10 according to the invention. Oneadvantage of water-based hydrostatic fluid is that if machining coolant(typically a water-based composition) leaks into or mixes with thehydrostatic fluid, it may not present a serious contamination problem.Water-based hydrostatic fluids may also be used in bearings 10 that areproduced for the food industry, because of the reduced risk ofcontaminating the consumable product. Additionally, water-based fluidsgenerally have high thermal conductivities, which enables the heatgenerated by the pumping process to be removed much more easily.

FIG. 3 is a schematic diagram of the vertical pads 26A and 28A, showingtheir basic geometry and illustrating the route fluid takes through thebearing pads 26A, 28A. Vertical bearing pad 26B is similar in design topad 26A and is therefore not shown. Vertical bearing pad 28B isidentical in design to 28A and is therefore not shown. In the followingdescription, it is assumed that the fluid path is the same in thenon-illustrated bearing pads 26B, 28B. However, as those of ordinaryskill in the art will realize, the design of the various verticalbearing pads 26A, 26B, 28A, 28B need not be identical.

A lubricating fluid is pressurized and supplied by pump 32 to the upperand lower bearing pads 26A and 28A. (The details of the hydraulic supplyof bearings 10 according to the invention will be described below withrespect to FIG. 18.) The fluid enters the lower pad 28A at supply groove34 which has a depth sufficient to allow free flow of fluid within it.Some fluid crosses leakage lands 36A and 36B, which are at a tight gapdistance from rail 14, and exits bearing pad 28A. Some fluid crossesland 38 and enters pocket groove 40. Some fluid also crossescompensating land 42 which is at a small distance from the rail 14; thistight gap creates a pressure drop as the fluid enters compensator groove44. Some fluid leaks from compensator groove 44 across lands 46A and 46Band exits bearing pad 26A. Some fluid is routed from compensator groove44 to pocket groove 48 of bearing pad 26A. Some fluid leaks out ofpocket groove 48 across lands 50A, 50B, and 50C where it exits bearingpad 26A. Fluid is free to flow in the tight gap region between rail 14,and central bearing pad 52, at a pressure that is equal to the fluidpressure in pocket groove 48. Fluid is also supplied at supply pressurefrom pump 32 to the supply grooves 54A and 54B of pad 26A. Some fluidleaks across lands 56A, 56B, 56C, and 56D and exits the bearing pad 26A.Some fluid crosses from supply grooves 54A and 54B across lands 58A and58B to pocket groove 48. Some fluid crosses from supply grooves 54A and54B across compensator lands 60A and 60B to compensator groove 62. Somefluid leaks from compensator groove 62, crosses land 64 and exitsbearing pad 26A. Some fluid is routed from compensator groove 62 tobearing pad 28A where it enters pocket groove 40. Some fluid then flowsfrom pocket groove 40 across lands 66A, 66B, and 66C where it exitsbearing pad 28A. Fluid can flow between compensator groove 44 and pocketgroove 40 but is largely restricted from doing so by land 68. Fluid canflow between compensator groove 62 and pocket groove 48 but is largelyrestricted from doing so by land 70.

Grooves 54A, 54B, 62, 48, 34, 44, and 40 all should have a depth that isat least about three times larger than the clearance between the pads28A and 26A and the rail 14 to ensure uniform pressure within each ofthese grooves. In the case of grooves 48 and 40, uniform pressure isdesired to spread the load-supporting pressure over the entire pocketarea. In the case of grooves 54A, 54B, 62, 34, and 44, uniform pressureis desired in order to yield the proper hydraulic resistance on theadjacent lands so that the pressure in the respective bearing areas canbe adequately controlled.

Pad 26A should be fabricated such that lands 52, 50A, 50B, 50C, 60A,60B, 56A, 56B, 56C, 56D, 64, 58A, 58B, and 70 are preferably all on thesame plane and at the same tight gap distance to rail 14. Pad 28A shouldbe fabricated such that lands 66A, 66B, 66C, 42, 46A, 46B, 36A, 36B, 38,and 68 are preferably all on the same plane and at the same tight gapdistance to rail 14.

FIG. 4 is a fluid circuit diagram of the vertical bearing pads 26A and28A (which are identical to the counterpart vertical bearing pads 26Band 28B). The various lands described above with respect to FIG. 3 areshown in FIG. 4 as circuit resistors. The values of the landresistances, which can be calculated by those skilled in the art offluid dynamics, is dependent upon the fluid viscosity, the length andwidth of the lands, and the clearance between each land and the rail 14.The fluid circuit shown in FIG. 4 can be solved by those skilled in theart of circuit analysis to compute the pressure in each of the bearinggrooves. These pressures may then be multiplied by the correspondingbearing areas to yield the overall vertical force developed by thebearing.

In order to evaluate how the bearing force changes in response to achange in vertical position of the carriage 12 with respect to the rail14, the fluid gap between the carriage 12 and the rail 14 that was usedto calculate the land resistances would be changed and the analysisdescribed above would be repeated with the new fluid gap data. Acomputer program could be used to carry out this repetitive analysis.Although the bearing pad geometries may be chosen to suit particularapplications of the hydrostatic bearing 10, it is preferable if the thebearing groove and land geometry are optimized to provide very highbearing stiffness and load capacity in the vertical direction with theminimum possible flow rate of fluid through the bearing 10 because highfluid flow rates typically require great amounts of pumping power.

Grooves 54A, 54B, 62, 48, 34, 44, and 40 are shown in FIG. 3 withrounded corners; however, they may be fabricated with sharp squarecorners or another geometric profile without considerable effect onbearing operation, since the hydraulic resistances of the adjacent landswill change by a very small percentage of their overall resistancevalues.

As shown in FIG. 3 and described above, fluid is routed between pad 28Aand pad 26A in two places, from compensator groove 44 to pocket groove48, and from compensator groove 62 to pocket groove 40. These fluidtransfers may be accomplished by the use of drilled holes in carriage 12and keeper 18A, or they may be accomplished with the use of rigid tubingexternal to carriage 12. Similarly, fluid may be routed at supplypressure from pump 32 to supply grooves 34, 54A, and 54B with the use ofexternal tubing followed by holes drilled in carriage 12 and keeper 18A.

FIG. 5 is a schematic view of the horizontal bearing pads 30A and 30B,showing their basic geometry and illustrating the route that fluid takesthrough the bearing pads 30A, 30B. A lubricating fluid is pressurizedand supplied by pump 32 to the upper and lower bearing pads 30A and 30B.(The same pump 32 may be used to supply the horizontal bearing pads 30A,30B and the vertical bearing pads 26A, 26B, 28A, 28B, or two differentpumps 32 may be used.) The fluid enters pad 30A at supply groove 72Awhich is at a depth sufficient to allow free flow of fluid within it.Some fluid leaks from supply groove 72A across leakage lands 74A and76A, which are at a tight gap distance from rail 14, and exits bearingpad 30A. Some fluid flows from supply groove 72A across lands 78AA and80AA to pocket groove 82AA and some flows across lands 78AB and 80AB topocket groove 82AB. Some fluid flows from supply groove 72A acrosscompensator lands 84AA, 86AA, 88AA to compensator groove 90AA. Some ofthe fluid which enters compensator groove 90AA leaks to or from pocketgroove 82AA across land 100AA. The remainder of the fluid which enterscompensator groove 90AA is routed to bearing pad 30B where it enterspocket groove 82BA and provides uniform pressure to pocket groove 82BAbefore leaking across lands 92BA, 94BA, and 96BA and exiting bearing pad30B. The fluid in the tight clearance of bearing pad 98BA will be at apressure equal to the fluid pressure in pocket groove 82BA becausepocket groove 82BA completely surrounds bearing pad 98BA. Fluid is alsosupplied at supply pressure from pump 32 to supply groove 72B of bearingpad 30B. Some of the fluid which enters supply groove 72B leaks acrosslands 74B and 76B and exits bearing pad 30B. Some of the fluid whichenters supply groove 72B leaks across lands 78BA and 80BA to pocketgroove 82BA, and some leaks across lands 78BB and 80BB to pocket groove82BB. Some of the fluid which enters supply groove 72B leaks acrosscompensator lands 84BA, 86BA, and 88BA to compensator groove 90BA. Somefluid may across land 100BA between compensator groove 90BA and pocketgroove 82BA. The remainder of fluid entering compensator groove 90BA isrouted to pad 30A where it enters pocket grooves 82AA and leaks acrosslands 92AA, 94AA, and 96AA and exits bearing pad 30A. The fluid in thetight gap clearance of bearing pad 98AA will be at a pressure equal tothe fluid pressure in pocket groove 82AA because pocket groove 82AAcompletely surrounds bearing pad 98AA. Some of the fluid which enterssupply groove 72A leaks across compensator lands 84AB, 86AB, and 88AB tocompensator groove 90AB. Some fluid may across land 100AB betweencompensator groove 90AB and pocket groove 82AB. The remainder of fluidentering compensator groove 90AB is routed to pad 30B where it enterspocket groove 82BB and leaks across lands 92BB, 94BB, and 96BB and exitsbearing pad 30B. The fluid in the tight gap clearance of bearing pad98BB will be at a pressure equal to the fluid pressure in pocket groove82BB because pocket groove 82BB completely surrounds bearing pad 98BB.Some of the fluid which enters supply groove 72B leaks acrosscompensator lands 84BB, 86BB, and 88BB to compensator groove 90BB. Somefluid may across land 100BB between compensator groove 90BB and pocketgroove 82BB. The remainder of fluid entering compensator groove 90BB isrouted to pad 30A where it enters pocket groove 82AB and leaks acrosslands 92AB, 94AB, and 96AB and exits bearing pad 30A. The fluid in thetight gap clearance of bearing pad 98AB will be at a pressure equal tothe fluid pressure in pocket groove 82AB because pocket groove 82ABcompletely surrounds bearing pad 98AB.

Grooves 82AA, 82AB, 82BA, 82BB, 90AA, 90AB, 90BA, 90BB, 72A, and 72B allshould have a depth that is at least three times larger than theclearance between the pads 30A and 30B and the rail 14 to ensure uniformpressure within each of these grooves. In the case of grooves 82AA,82AB, 82BA, and 82BB, uniform pressure is desired in order to spread theload-supporting pressure over the entire pocket area. In the case ofgrooves 90AA, 90AB, 90BA, 90BB, 72A, and 72B, uniform pressure isdesired in order to yield the proper hydraulic resistance on theadjacent lands so that the pressure in the respective bearing areas canbe adequately controlled.

Pad 30A is fabricated such that lands 98AA, 98AB, 84AA, 84AB, 86AA,86AB, 88AA, 88AB, 92AA, 92AB, 94AA, 94AB, 96AA, 96AB, 78AA, 78AB, 80AA,80AB, 100AA, 100AB, 74AA, 74AB, 76AA, 76AB are preferably all on thesame plane and at the same tight gap distance to rail 14. Pad 30B shouldbe fabricated such that lands 98BA, 98BB, 84BA, 84BB, 86BA, 86BB, 88BA,88BB, 92BA, 92BB, 94BA, 94BB, 96BA, 96BB, 78BA, 78BB, 80BA, 80BB, 100BA,100BB, 74BA, 74BB, 76BA, 76BB are preferably all on the same plane andat the same tight gap distance to rail 14.

FIG. 6 is a schematic diagram showing the fluid resistances of thehorizontal bearing pad 30A. Each of the resistances shown in FIG. 6represents one of the lands of the horizontal bearing pad 5A. The valuesof the resistances of the horizontal bearing pad 30A may be calculatedas was described above with respect to the vertical bearing pads 26A,26B, 28A, 28B.

Grooves 82AA, 82AB, 82BA, 82BB, 90AA, 90AB, 90BA, 90BB, 72A, and 72B areshown in FIG. 5 with rounded corners; however, they may be fabricatedwith sharp square corners or another geometric profile withoutconsiderable effect on bearing operation since the hydraulic resistancesof the adjacent lands will change by a very small percentage of theiroverall resistance values.

As shown in FIG. 5, fluid is routed between pad 30A and pad 30B in fourplaces: from compensator groove 90AB to pocket groove 82BB, fromcompensator groove 90AA to pocket groove 82BA, from compensator groove90BA to pocket groove 82AA, and from compensator groove 90BB to pocketgroove 82AB. As with the fluid transfers in the vertical bearing pads26A, 26B, 28A, 28B, these fluid transfers may be accomplished by the useof drilled holes in carriage 12, or they may be accomplished with theuse of rigid tubing external to carriage 12. Similarly, fluid may berouted at supply pressure from pump 32 to supply grooves 72A and 72Bwith the use of external tubing followed by holes drilled in carriage12.

In the vertical and horizontal bearing pads shown in FIGS. 3 and 5 anddescribed above, lands 58A, 58B, 70, 38, 68, 78AA, 78AB, 78BA, 78BB,80AA, 80AB, 80BA, 80BB, 100AA, 100AB, 100BA, and 100BB allow leakagepaths between adjacent compensators, pockets, and supply grooves. Theseleakage paths tend to reduce the pressure response of the bearing andtherefore reduce its stiffness and load-carrying capability. However, agreater factor that overcomes the effect of these fluid leakage paths isthe ability to arrange pocket grooves 48, 40, 82AA, 82AB, 82BA, and 82BBsuch that they are closer to the compensating grooves, and, therefore,spread the load-supporting pocket pressures over a larger area. Bybetter utilizing the available pad area, the bearing pad configurationsof the hydrostatic bearing 10 provide higher stiffness and loadcapacity.

FIG. 7 is another perspective view of the hydrostatic bearing of FIG. 1,with its seals and endcaps installed. FIG. 8 is a sectional view throughLine 8-8 of FIG. 7, and FIG. 9 is a close-up view of portion A (enclosedin dotted line) of FIG. 8. FIGS. 7-9 show the hydrostatic bearing ofFIG. 1 with end caps 102A and 102B attached to carriage 12 and keepers18A and 18B. End caps 102A and 102B contain reservoirs 104A and 104B(visible in the views of FIGS. 8 and 9) to which the fluid flows intofrom bearing pads 26A, 26B, 28A, 28B, 30A, and 30B as well as from draingrooves 106, 108A, 108B, 110A, 110B, 112A, and 112B. (As was describedabove, the drain grooves are provided at the corners of the carriage 12and are visible in the views of FIGS. 1 and 2.) Double-lipped end seals114A and 114B are attached to end caps 102A and 102B. The double-lippedend seals 114A, 114B are attached to rigid plates 113A, 113B in order toprovide them with additional stiffness. Lips 116 of end seals 114A and114B are in sliding engagement with rail 14 and serve to trap the fluidinto reservoirs 104A and 104B and largely prevent fluid from leakingdirectly out of the hydrostatic bearing 10. The fluid flows out ofreservoirs 104A or 104B through at least one drain outlet 118A and/or118B. One or more of the drain outlets 118A, 118B may be plugged, but atleast one drain outlet 118A, 118B is used to route the fluid to a hoseor tubing assembly, where the fluid is returned to the hydraulic supplysource.

FIG. 10 is a sectional side elevational view of the hydrostatic bearing10, illustrating side seals 120A and 120B that are received by acceptorgrooves 122A and 122B within keeper portions 18A and 18B of the bearingcarriage 12. FIG. 11 is an enlarged sectional view of portion B of FIG.10, illustrating the side seals 120A, 120B in more detail. The sideseals 120A, 120B slidingly engage the bearing rail 14, serve to trapfluid, and allow the trapped fluid to be routed through drain grooves112A and 112B into reservoirs 104A and 104B to prevent fluid fromleaking directly out of the hydrostatic bearing 10. As shown in FIG. 11,the side seals 120A, 120B have a generally u-shaped portion 121 thatopens upwardly, towards the top of the drain groove 112A, 112B. The sideseals 120A, 120B are positioned in the acceptor groove 122A, 122B suchthat one wall of the u-shaped portion 121 of the side seal 120A, 120B isin contact with the keeper 18A, 18B and the other wall of the u-shapedportion 121 is in contact with the bearing rail 14.

FIG. 13 is a perspective view of the underside of the central portion 16of the carriage 12 without the keepers 18A, 18B installed. FIG. 13 showsthe relative locations and extents of the vertical bearing pads 26A, 26Band the horizontal bearing pads 30A, 30B. FIGS. 14 and 15 areperspective views of the keepers 18A and 18B, showing the locations andextents of vertical bearing pads 28A and 28B on the keepers 18A and 18B.The positions of the drain grooves 106, 108A, 108B, 110A, 110B, 112A,and 112B and seal acceptor grooves 122A, 122B are also shown.

Each side of the central portion 16 of the bearing carriage 12 has a setof threaded holes 222 provided in respective connecting surfaces 220Aand 220B. A set of complimentary, counterbored through holes 226 areprovided in the keepers 18A and 18B. When the keepers 18A and 18B andcentral portion 16 of the carriage 12 are assembled, bolts are insertedthrough the holes 226 in the keepers 18A, 18B and into the threadedholes 222 of the central portion 16 of the carriage 12 such that theengaging surfaces 220A, 220B of the central portion 16 and the engagingsurfaces 224A, 224B of the keepers 18A, 18B are adjacent, as shown inFIG. 10.

The bearing pad grooves and other surface features shown in FIGS. 13-15may be formed by milling, electrical discharge machining, or other knowntechniques.

FIG. 16 is a perspective view showing the end seals 114A, 114B and sideseals 120A, 120B in isolation. As was described above, the end seals114A, 114B are constructed of a rubber material molded so as to attachto rigid plates 113A, 113B, for example, steel or aluminum plates, toprovide them with greater rigidity. In alternative embodiments, the endseals 114A, 114B may not be attached to rigid plates 113A, 113B

As is shown best in FIG. 17, a close-up perspective view of portion “C”of FIG. 16, the side seals 120A, 120B are inserted into receptacles 115formed in the end seals 114A, 114B such that they have an interferencefit with the receptacles 115. In one embodiment, the side seals 120A,120B may be made slightly longer than required, such that they can bemaintained in compression during operation. In alternative embodimentsof the invention, the side seals 120A, 120B and the end seals 114A, 114Bmay be molded or cast as a single structure, bonded together, orotherwise caused to adhere to one another to form a unitary structure.

The bearing pads 26A, 26B, 28A, 28B, 30A, 30B described above aredesigned for a self-compensating hydrostatic bearing. However, those ofordinary skill in the art will realize that the other features of thecarriage 12 and rail 14, including the sealing structures (i.e., the endseals 114A, 114B and side seals 120A, 120B) and the drain grooves 106,108A, 108B, 110A, 110B, 112A, and 112B may be used without theparticular bearing pads 26A, 26B, 28A, 28B, 30A, 30B described above.For example, in alternative embodiments of the invention, a carriagehaving end seals, side seals and a drain groove arrangement similar tothat described above could be used with bearing pads that are notself-compensating. Bearing pads that are not self-compensating could usecapillary tubes or valves for compensation purposes, as one of ordinaryskill in the art will readily be able to appreciate.

Conversely, the self-compensating bearing pads 26A, 26B, 28A, 28B, 30A,30B described above may be used on other types of hydrostaticallysupported devices and in other types of fluidstatic bearings without theother features described herein.

FIG. 18 is a schematic perspective view illustrating four bearingcarriages 12 riding on two carriage rails 14. In general, severalbearing carriages 12 may be provided on the same carriage rail 14,particularly if those bearing carriages 12 are fixed in position withrespect to one another (e.g., by being bolted to the bed of a machinetool, as will be described below). Alternatively, several shortersegments of bearing rail 14 could be provided, one segment for eachbearing carriage 12.

FIG. 18 also illustrates the details of the hydraulic fluid connectionsfor the bearings 10 according to the present invention. A hydraulicpower unit 230 delivers hydraulic fluid under high pressure through aconduit 232. The hydraulic power unit 230 includes all of the componentsnecessary to deliver temperature controlled fluid that is relativelyfree of contaminant particles at high pressure with minimal pressurepulsations. For example, the hydraulic power unit 230 may include areservoir, a pump, an electric motor, a filter, a pressure regulatingvalve, a pressure gauge, and a heat rejection system, such as anair-to-oil heat exchanger.

The conduit 232 from the hydraulic power unit 230 branches such that onebranch connects with each bearing carriage 12. The branches of theconduit 232 are received by a fluid inlets 119 in the end seals 114A,114B of the bearing carriages 12. (Depending on the configuration of thebearings 10, the conduit 232 may connect to a fluid inlet 119 on eitherend seal 114A, 114B. The unused fluid inlet 119 may be plugged oromitted.) The connection between the conduit 232 branch and the fluidinlet 119 of the end seal may be any appropriate type of conventionalhydraulic connection. From the fluid inlet 119, the pressurized fluid isdistributed to the supply grooves 34, 54A, 54B by an internal network ofpassageways. Once used, the fluid is collected in the reservoirs 104A,104B and returned via return conduits 238, which connect to the drainoutlets 118A, 118B and the return portions of the hydraulic power unit232.

FIG. 12 is a side elevational view of a machine tool 200, illustrating atypical application for a hydrostatic bearing 10 according to thepresent invention. A machine tool table 66 is supported by four bearingassemblies 10 which ride on two rails 14. Although only two bearingassemblies 10 are shown, at least four are typically used to provideadequate pitch and yaw stability to table 202. The rails 14 of thehydrostatic bearings 10 are horizontally clamped to a machine bed 204using wedges 206A and 206B. The rails 14 are clamped vertically tomachine bed 204 using a plurality of bolts 208 threadedly secured withinmachine bed 204 through counterbored holes 20 provided in the rail 14.Two of the hydrostatic bearings 10 are clamped horizontally to the table202 using wedges 210 (one wedge 210 is shown in the view of FIG. 12).The other two hydrostatic bearings 10 are floated into alignment bypressurizing them with lubricating fluid, thus allowing hydrostaticbearings 10 to float horizontally into a self-aligning position. Oncethe two wedge-secured hydrostatic bearings 10 are in alignment, thebolts that secure them to the table 202 are tightened. Although FIG. 12illustrates the use of wedges 206A, 206B, and 210, many other mechanismsto clamp the rails 14 and the hydrostatic bearings 10 are possible andare within the scope of the invention.

A hydrostatic bearing 10 may be used in a number of different types ofmachine tools, and in any other application in which linear motionguidance is required. However, hydrostatic bearings 10 according to theinvention may be particularly beneficial when used in lathes. Forexample, hydrostatic bearings 10 may be used in the QUEST® turningmachines manufactured by HARDINGE, Inc. (Elmira, N.Y., United States).Hydrostatic bearings 10 may also be useful in grinding machines, millingmachines, boring machines, and other machine tools in which acombination of high stiffness and damping are beneficial.

A hydrostatic bearing 10 according to the present invention may havecertain advantageous performance characteristics. For example, ahydrostatic bearing 10 according to the invention would typically havehigh static and dynamic stiffnesses. A hydrostatic bearing 10 may alsooperate with very low friction, because the seals described above withrespect to FIGS. 7-11 would generally be the only components creatingfriction. Because the carriage 12 rides on a layer of fluid, and forother reasons, the hydrostatic bearing 10 may have up to ten times theforce damping capabilities of a conventional rolling element linearbearing. Additional advantages may include an essentially unlimitedtranslational (feed) rate, an essentially unlimited fatigue life (withsubstantially no component wear because the carriage 12 and rail 14 arenot in contact), substantially no change in positioning accuracy of amachine tool mounted on hydrostatic bearings 10 over time, substantiallyno damage to the hydrostatic bearing 10 under heavy “crash” loads (i.e.,when the bearing 10 stops suddenly at the ends of its travel range).Moreover, the hydrostatic bearing 10 is self cleaning if fluid flow ismaintained between the carriage 12 and rail continuously 14.

When installed in a machine tool and used to produce parts, the featuresof the hydrostatic bearing 10 may also lead to certain other advantages.For example, the hydrostatic bearing 10 may improve tool life.Additionally, parts may be produced with better surface finishes andbetter roundnesses for round parts. A machine tool mounted onhydrostatic bearings 10 may also have improved hard turning capability,improved interrupted cutting capability, and improved positioningaccuracy. Some of the advantages and benefits described above willbecome apparent from the following example.

EXAMPLE 1

A hydrostatic bearing 10 according to the invention is installed so asto support operational movement in a QUEST® 51 turning machine(Hardinge, Inc., Elmira, N.Y., United States) using the installationprocedure described above. Four hydrostatic bearings 10 according to thepresent invention are installed to guide motion in the X-axis and fourare installed to guide motion in the Z-axis. No adaptations to theturning machine are required in order to accommodate the hydrostaticbearings 10; however, hydraulic hoses are provided for each hydrostaticbearing 10. A two-inch round A2 tool steel blank was prepared with fourslots milled around its circumference for interrupted cutting. It wasthen hardened to 60-62 Rc. The part was then roughed with a 5/16 inchdiameter round cubic boron nitride (CBN) insert at 450 SFM/0.002ipr/0.030 doc with five passes. Subsequently, the part was finished witha 55 degree CBN insert at 550 SFM/0.003 ipr/0.005 doc with one pass, andthen threaded with a CBN triangular insert. The surface finish of thepart was consistently in the 5 to 6 microinch range, an improvement ofapproximately a factor of two when compared with an identical partmachined on a comparable QUEST® 51 turning machine without a hydrostaticbearing. Additionally, the tool life of the interrupted turning insertwas increased by a factor of three when compared to the life of aninsert used on the turning machine without the hydrostatic bearing.

Although the invention has been described with respect to certainembodiments, those embodiments are intended to be illustrative, ratherthan limiting. Modifications and variations to the invention arepossible, within the scope of the appended claims.

1. A bearing carriage, comprising: one or more bearing pads constructedand arranged to receive fluid from a pressurized fluid source and tocause that fluid to flow selectively over a collection of bearinggrooves and resistive lands so as to create a supporting fluid layerbetween said bearing carriage and a structure on which said bearingcarriage is mounted for movement; and a fluid recovery systemconstructed and arranged to prevent fluid from flowing out of the spacebetween said bearing carriage and the structure on which said bearingcarriage is mounted for movement and to route the fluid back towards thepressurized fluid source, said fluid recovery system comprising: asealing structure having contiguous end and side portions, said endportions being constructed and arranged to seal ends of said bearingcarriage and said side portions being constructed and arranged to extendalong at least a portion of sides of said bearing carriage to seal saidsides, said end portions including a double-lipped seal, a first lip ofthe double-lipped seal engaging the structure on which said bearingcarriage is mounted for movement and a second lip of said double-lippedseal preventing debris from entering said bearing carriage; reservoirstructures defined by portions of said bearing carriage and sealed bysaid sealing structure; and drain grooves constructed and arranged toconduct pressurized fluid from said bearing pads to said reservoirstructures.
 2. The bearing carriage of claim 1, wherein said draingrooves are between said bearing pads and the side portions of saidsealing structure.
 3. A hydrostatic bearing carriage constructed andarranged to be mounted for hydrostatically supported movement on abearing rail, said bearing carriage comprising a plurality ofself-compensating bearing pads, said plurality of bearing pads beingconstructed and arranged to be in fluid communication with a pressurizedfluid source, each of the bearing pads comprising: a compensatinggroove; a supply groove fluidly connected to a compensating groove onanother of the plurality of bearing pads, the supply groove beingconstructed and arranged to receive pressurized fluid from thecompensating groove on another of the plurality of bearing pads; and aresistive land surrounding the groove and constructed and arranged toreceive pressurized fluid from the compensating groove to create asupporting fluid layer between the bearing pad and the bearing rail,wherein the resistive land is entirely planar.
 4. The hydrostaticbearing carriage of claim 3, further comprising: a fluid return systemincluding at least one drain groove completely surrounding the resistivelands; and a seal structure completely surrounding the at least onedrain groove.